Electrical wave guide provided with tuning pistons



April 13, 1954 CQLLARD 2,675,524

ELECTRICAL wAvE GUIDE PROVIDED WITH TUNING PISTONS Filed March 16, 1949 Afro/we) Patented Apr. 13, 1954 ELECTRICAL WAVE GUIDE PROVIDED WITH TUNING PISTONS John Collard, Hammersmith, London, England,

assignor to Electric & Musical Industries Limited, Hayes, England, a British company Application March 16, 1949, Serial No. 81,656

Claims priority, application Great Britain March 25, 1948 4Claims.

This invention relates to electrical waveguides of the kind having a rectangular or other noncireular cross-section. For tuning such waveguides it has been the practice to employ a piston having a shape conforming to the cross-sectional area of the waveguide and to cause the piston to move axially along the waveguide by the provision of a suitable screw. In general it is difficult tomake the piston a perfect fit in the waveguide and it is usually therefore the practice to provide the piston with some form of wave trap toprevent high-frequency power from passing the piston.

A piston for use in a non-circular waveguide is difficult to construct and, moreover, since it is guided by the sides of the waveguide which are not usually verysmooth the travel of the piston is often erratic and, furthermore, since it is usual to employ a-screw to cause axial movement of the piston it is necessary, since the piston cannot rotate relatively to the waveguide to provide a joint. between the screw and the piston to allow the piston to move axially without rotating. The provision. of such a joint, however, often introduces back-lash.

It has hitherto as will be appreciated from the above, been considered necessary to make the piston conform as closely as possible to the wall of the waveguide. I have found however, that the close fit, contrary to the practice hitherto, is unnecessary since providing the piston is arranged close to the walls of the waveguide where maximum voltage exists a wide separation between the piston and the other walls of the waveguide can be employed. For example with a waveguide of rectangular cross-section and excited in the H mode, the maximum voltage exists at the mid-point of the longer walls of the waveguide cross-section. In such a case the piston can be arranged close to the longer walls but separated widely from the shorter walls. This leads to considerable simplification of the construction of tuning piston.

Thus when using non-circular waveguides it is not necessary to employ a piston having a shape corresponding to the cross-sectional shape of the waveguide since, providing the piston is of a suiiicient cross-sectional area to cause attenuation of the waves within the free space between the piston and the wall of the waveguide, the piston can be of any shape in cross-section and can therefore be of such a shape that it can rotate within the waveguide and hence avoid the necessity of providing a joint which permits relative With such a form of piston it is necessary'to provide the latter with some form of wave trap to prevent power from passing through the coaxial circuit which is formed by the piston and the waveguide.

Acording therefore to one feature of the invention there is provided a waveguide of non-circular cross-section having a tuning piston of such a shape that it can rotate within the waveguide and having a cross-sectional area to cause attenuation of waves within the free space and between the piston and the wall of the waveguide, said piston also having means for preventing power from passing along the circuit formed by the piston and the waveguide.

According to another feature of the invention there is provided a waveguide having a rectangular cross-section and having a tuning piston of circular form in cross-section so that it can rotate within the waveguide and having a cross-sectional area to cause attenuation of waves within the free space between the piston and the wall of the waveguide, said piston also having means for preventing power from passing along the circuit formed by said piston and waveguide.

In order that the said invention may be clearly understood and readily carried into effect, it will now be more fully described with reference to the accompanying drawings, in which:

Figure 1 is a transverse section through a waveguide and a tuning piston arranged therein,

Figure 2 is a cross-sectional view takenal'ong the line 22 of Figure 1, and

Figure 3 is a View similar to Figure 2 showing a modified form of piston.

As shown in Figure 1, the reference numeral 1 indicates a waveguide of rectangular cross-section, the longer dimension of the cross-sectional area of the waveguide being chosen so as to pass power at a desired operating frequency and usually in the H10 mode so that the electric vector will be directed perpendicularly to the longer Walls of the waveguide as seen in cross-section. In order to tune the waveguide a tuning piston 2 is employed which is of circular cylindrical form, as shown, and having a diameter slightly less than the smaller dimension of the waveguide cross-section, the piston being disposed centrally between the walls of the waveguide. The piston is provided with a suitable adjusting screw 3 supported in any convenient manner relatively to the waveguide. Owing to the cylindrical construction of the piston it will be appreciated that the latter on rotation of the screw 3 can rotate movement between the screw and the piston. 56 within the waveguide and move axially with respect thereto for tuning purposes and out of contact with all of the walls of the waveguide. The free space which is left surrounding the piston is too small to pass power at the operating frequencyand hence the piston acts as a reflecting surface. However, the piston 2 and the waveguide form a coaxial circuit the piston forming the centre conductor and wall of the waveguide the outer conductor of said circuit. In order to prevent power from passing through the coaxial circuit as formed, the circumferential surface of the piston 2 is provided with one or more grooves 4 having a depth of a quarter of wavelength at the operating frequency and if more than one of said grooves are provided, as shown in Figures 2 and 3, successive grooves are spaced apart a quarter of an operating wavelength. Each groove 4 presents at its open end a high impedance which is transformed to a low impedance bythe preceding length of piston and thus presents a low impedance and hence an effective short circuit between the piston and the wall of the waveguide.

As the diameter of the piston 2 may be such that with grooves a quarter of wavelength deep th bottoms of the grooves may merge into one another, the grooves 4 as shown in Figure 2 are disposed obliquely to the longitudinal axis of the piston so as to avoid such merging. Alternatively, as shown in Figure 3, the grooves can be disposed in planes at right-angles to the longitudinal axis of said piston and the grooves filled with a suitable dielectric materal, such as for example, a phenolic synthetic resin so that the wavelength in the dielectric material is less than in air whereby the depth of the grooves can be reduced.

The length of the piston and the number of grooves employed may be varied to suit particular circumstances. If the piston is used in a Wavemeter having a high Q it will preferably be desirable to employ three or more grooves, whereas for a simple reactance tuner in a matched wave guide a single groove will usually be sufficient.

Although the invention has been described as applied to guides having a rectangular crossseetion and employing a piston of circular form in cross-section, it will be appreciated that the invention is not limited thereto since it can be employed with waveguides having other noncircular forms of cross-section and to pistons of other than circular form.

What I claim is:

1. A waveguide of rectangular cross section and excitable in the H mode, and a tuning piston, means rotatably mounting said piston for axial movementinthe Waveguide with its axis of rotation parallel to the longitudinal axis of the waveguide, said piston having a circular form in cross section and disposed centrally between the walls of the waveguide which are disposed closer together and having a diameter slightly less than the smaller dimension of the waveguide cross section, said piston having means for effectively short circuiting the piston to said walls of said wave guide.

2. A waveguide of rectangular cross section and excitable in the H10 mode, a tuning piston, means rotatably mounting said piston for axial movement in the waveguide with its axis of rotation parallel to the longitudinal axis of the waveguide, said piston having a circular form in cross section and disposed centrally between the walls of the waveguide which are disposed closer together and having a diameter slightly less than the smaller dimension of the waveguide cross section, said piston having means for effectively short circuiting the piston to said walls of said Wave guide, and a screw-threaded rod fixed to said piston and engaging a screwthreaded support for rotating the piston to move it axially of said waveguide with the axis of rotation of said piston parallel to the longitudinal axis of the waveguide.

3. A waveguide of elongated cross-section having a tuning piston rotatable within the waveguide, said piston having such cross-sectional area in relation to the internal dimensions of the waveguide that a pair of opposite surfaces on said piston are disposed close to but out of contact with areas adjacent said piston on the internal surface of said waveguide Where maximum voltage exists, other surfaces on said piston being more widely spaced from said waveguide, said piston also having means for effectively short-circuiting the piston "to said areas of said waveguide, an a screw-threaded rod fixed to said piston and engaging a screw-threaded support for rotating the piston to move it axially of said waveguide with the axis of rotation of said piston parallel to the longitudinal axis of the waveguide.

4. A waveguide of elongated cross-sectionhaving a tuning piston rotatable within the waveguide, said piston having such cross-sectional area in relation to the internal dimension of the waveguide that a pair of opposite surfaces on said piston are disposed close to but out of contact with areas adjacent said piston on the internal surface of said waveguide where maxi.- mum voltage exists, other surfaces on said piston being more widely spaced from said waveguide, said piston having a circumferential groove spaced from one end of said piston to provide an effective short circuit between the piston and the said areas of said waveguide, and a screwthreaded rod fixed to said piston and engaging a screw-threaded support for rotating the piston to move it axially of said waveguide with the axis of rotation of said piston parallel to the longitudinal axis of the waveguide.

References Cited in the file 6r this patent UNITED STATES PATENTS Number Name Date Re. 23,131 Webber June 28, 1949 2,106,769 Southworth Feb. 1, 1938 2,441,598 Robertson May 18, 1948 1 2,451,876 Salisbury Oct. 19, 1948 2,518,092 Sunstein et a1; Aug. 8; 1950 2,530,248 Larson Nov. 14, 1950 2,543,721 Collard et al. Feb. 27, 1951 2,548,655 Cutler 'Apr. 10, 1951 

